1. Field of the Invention
The present invention relates to a rotor blade of a gas turbine used for a thermal power plant, etc., and more specifically to a rotor blade of a gas turbine in which a plurality of cooling holes are bored for the flow of a cooling gas.
2. Description of the Prior Art
FIG. 2 shows a prior art rotor blade of a gas turbine used for a thermal power plant, etc. This rotor blade is called an integral shroud blade and there is formed a shroud 11 integrally with a blade portion 12 at a tip of the blade portion 12.
The shroud 11 serves to reduce the amount of gas leaking from the tip portion of the blade portion 12 as well as to enhance the vibration resistance strength of the blade portion 12 as an end face of the shroud 11 is pressure-welded to an end face of a shroud 11 of another adjacent blade so as to form a series of grouped blades. If a two-directional vibration in a rotation-axial direction and a circumferential direction occurs at the blade portion 12, the end face of the shroud 11 is formed inclined and the adjacent shrouds are pressure-welded together at the inclined end faces, and thus the two-directional vibration is suppressed.
Also, in order to reduce the amount of gas leaking from the tip portion of the blade portion 12 as well as to prevent the shroud 11 from coming into contact with a casing side, there is provided a fin 17 to the shroud 11 by cutting.
In this type of gas turbine rotor blade, in order to correspond to a high temperature gas, there are provided a multiplicity of cooling holes 13 so as to carry out convection cooling in the blade portion 12. Further, the shroud 11 is made thinner so that its plane forms a shape like a ray fish.
Also, in the shroud 11, there is framed along the tip portion of the blade portion 12 a two-step groove 14 of a shape such that a groove is formed so as to communicate with the cooling holes 13. A plurality of cooling holes 15 for cooling the shroud 11 are bored from an end portion of the shroud 11 toward the two-step groove 14 along a face direction of the shroud 11.
In boring the cooling holes 15 in the shroud 11, the two-step groove 14 is formed in advance in the shroud along outlets of the cooling holes 13 of the blade portion 12. The cooling holes 15 are bored toward the two-step groove 14 in the shroud 11, and then an upper groove of the two-step groove 14 is covered by a plate-like plug 16. This plug 16 is inserted into the upper groove of the two-step groove 14 to a depth so as not to plug the cooling holes 15 of the shroud 11, and is then fixed by welding or the like around its periphery.
A cooling gas flows through the cooling holes 13 to cool the blade portion 12, flows through the cooling holes 15 to cool the shroud 11, and then flows out of the end portion of the shroud 11. As the cooling holes 13 of the blade portion 12 so communicate with the cooling holes 15 of the shroud 11, the cooling gas is efficiently consumed. Also, because the two-step groove 14 is formed in the shroud 11, working of the cooling holes 15 in the shroud 11 is facilitated.
As mentioned above, the present gas turbine rotor blade is alleviated in weight of the shroud 11, thus a remarkably small stress is acting centrifugally on a root portion of the shroud 11 so that the life of the rotor blade is elongated. Also, the cooling gas which has flown through the cooling holes 13 of the blade portion 12 flows through the cooling holes 15 of the shroud 11 and is discharged, and thus the shroud 11 is well cooled so as to reduce a temperature of the shroud 11, and the life of the rotor blade is further elongated.
Also, in boring the plurality of cooling holes 15 in the shroud 11, the two-step groove 14 is formed in advance along outlets of the cooling holes 13 of the blade portion 12 The work is done merely by boring the cooling holes 15 toward the two-step groove 14 in the shroud 11, and hence the boring of the cooling holes 15 is facilitated.
As mentioned, there are various advantages in the present gas turbine rotor blade. However, with respect to the consumption of cooling gas which has a small influence on gas turbine performance, the cooling gas flows out of each of the first cooling holes 13 to join together in the two-step groove 14, and then flows into each of the second cooling holes 15 bored toward both side faces of the shroud 11.
Accordingly, corresponding to a difference in a resistance of each of the second cooling holes 15 which communicate with the two-step groove 14, a difference in flow rate of the cooling gas which flows in each of the cooling holes 15 occurs resulting in portion of a difference in cooling effect, so that a uniform distribution control cannot be effected.